The liquid crystal display device is a display device using a liquid composition. The liquid crystal display device is widely used as a display device in various apparatuses, particularly, as an information display device and an image display device. The liquid crystal display device displays an image by transmitting or blocking light for each region on the basis of application of a voltage. Accordingly, external light is necessary for display of an image on the liquid crystal display device. Accordingly, a backlight, which is provided on a rear surface of the liquid crystal display device, is used as a light source. The backlight is used to irradiate the entirety of the liquid crystal display device with white light, and a cold-cathode tube has been used as a backlight light source in the related art. Recently, a light-emitting diode (LED) is also used instead of the cold-cathode tube for the reason of a long operational lifespan, excellent coloring, and the like.
However, recently, a nano-sized fluorescent substance using a quantum dot is made into a product mainly by a venture company at abroad. The quantum dot is a conductive crystal which is formed to trap an electron in a minute space and has a size of several nanometers or less. When being trapped by the quantum dot, the electron only exists as a standing wave with a discrete wavelength. Therefore, energy of the electron becomes discrete. When the quantum dot emits and absorbs a photon, an energy level of the electron varies. In addition, the energy of the photon, which is emitted or absorbed by the quantum dot, varies depending on a crystal size of the quantum dot (quantum size effect). Accordingly, quantum dots having various fluorescent colors can be manufactured. The fluorescent substance composed of the quantum dot is used, for example, as a fluorescent marker in a biotechnology.
As a method of synthesizing the quantum dot, for example, there is known a method in which a precursor-containing solution for formation of particles is continuously supplied to the inside of a reactor disposed in a heating zone, rapid heating is carried out up to a reaction initiation temperature, and rapid cooling is carried out after allowing a reaction to occur (for example, refer to Patent Literature 1 and Patent Literature 2).
However, in the majority of LEDs for both a backlight and illumination, a blue LED is covered with a fluorescent substance for fluorescent light-emission, and a desired light-emission color is obtained by using the light-emission. In the related art, as the fluorescent substance, dope-type light-emitting particles, which contain an oxide as a main component and has a size of several micrometers to several tens of micrometers, are used. A particle size of the fluorescent substance is as large as 10 or more times a wavelength of visible light (approximately 360 nm to 830 nm), and thus there is a problem in that scattering of light is likely to occur in a particle surface of the fluorescent substance, and an energy loss occurs. In addition, only a surface of the fluorescent substance contributes a wavelength variation of light, and thus there is also a problem in that optical conversion for the volume of the fluorescent substance particles is not efficient.
In addition, with regard to the dope-type light-emitting particles, there is a problem in that a light-emitting wavelength, light-emitting intensity, and temperature dependency are approximately fixed in accordance with a kind of a dopant (light-emitting ion, atom). Further, there is a problem in that a material capable of obtaining light-emitting intensity for practical use is limited. In addition, an excitation wavelength and a fluorescent wavelength correspond to each other in a one-to-one manner. Accordingly, a fluorescent substance capable of being used at one excitation wavelength is limited, and thus there is a problem in that the degree of freedom of a color tone is low.
As described above, the fluorescent substance of the related art has a lot of problems when being used as an optical material. As a result, when using the fluorescent substance of the related art, the following problems are caused in a liquid crystal display device.
First, conversion efficiency into exciting light in a fluorescent substance, and extraction efficiency of the fluorescent substance are poor, and thus an energy loss increases. In addition, spectral characteristics of a color filter in a liquid crystal display device, and a wavelength of fluorescence do not match each other, and thus an additional energy loss occurs. Accordingly, power consumption of the liquid crystal display device increases more than necessary.
On the other hand, a liquid crystal display device, which uses the quantum dot as a fluorescent substance layer, is also suggested. For example, Patent Literature 4 suggests a liquid crystal display device in which the quantum dot is used as the fluorescent substance layer, and which has a small optical loss. In the liquid crystal display device described in Patent Literature 4, light from a light-emitting diode is incident to a backlight (a light-guiding plate), and light, which is scattered in the backlight, is incident to a layer including the quantum dot fluorescent substance and is converted to white light by using optical wavelength conversion characteristics of the quantum dot. The white light that is obtained is used to irradiate a liquid crystal panel to obtain color image information.
In this method, an energy loss is smaller in comparison to a liquid crystal display device of the related art in which large-sized dope-type light-emitting particles are used as a fluorescent substance, and it is possible to manufacture a liquid crystal display device with satisfactory color reproduction. However, in the method, it is necessary to form a fluorescent substance layer with a wide area similar to the backlight of the related art. Accordingly, the following problems and the like still exist. Specifically, it is necessary to use a lot of quantum dot fluorescent substances, it is necessary to apply a fluorescent substance to a wide area in a uniform concentration, and it is difficult to make the physical thickness of the liquid crystal display device small.
As a method of making the thickness of the backlight small, there is also suggested a method of reducing the thickness by employing a configuration in which light is incident from an end surface of the backlight. In Patent Literature 5, a plurality of light sources with light-emitting colors different from each other are used, and the light sources are disposed in a region (short side) having a narrow end surface instead of a wide surface of the backlight, thereby making the thickness of the device small.
However, in the method of Patent Literature 5, it is difficult to use a light source with a small size and high efficiency, and thus a cold-cathode tube is used. In addition, Patent Literature 5 also describes a configuration in which light-emitting diode light sources having a plurality of colors are used, but a specific structure thereof is not described. Patent Literature 5 discloses a method in which each color of light source is disposed at each of three short sides of the backlight, but in the method, three sides of the backlight are used as a light source, and three kinds of light sources are necessary. Accordingly, it is possible to make the periphery of the liquid crystal display device in a planar direction narrow, but the backlight becomes expensive.
In addition, in the fluorescent substance layer obtained by the method of using the dope-type fluorescent substance particles in the related art, the size of the fluorescent substance particles is as large as 10 or more times the wavelength of visible light (approximately 360 nm to 830 nm), and thus scattering and reflection of light due to the fluorescent substance particles are great. Accordingly, an optical loss at the fluorescent substance layer is large. Therefore, it is necessary to emit strong light so as to compensate the optical loss due to the fluorescent substance layer, and thus there are problems in that an operational lifespan shortens due to an increase in heat generation in the light-emitting diode, an operational lifespan of the fluorescent substance layer shortens due to exposition of the fluorescent substance to the strong LED light, and the like. Accordingly, it is difficult to provide the fluorescent substance layer in a region with a narrow area which is close to the LED. In the dope-type light-emitting element, a light-emitting wavelength and the like are fixed in accordance with the kind of a dopant, and thus it is difficult to allow the excitation wavelength and the light-emitting wavelength to match each other, and as a result, light-emitting efficiency is low. Particularly, this problem becomes significant in the vicinity of red light.